A conventional electric power steering apparatus of such a type includes an electric motor for applying steering assist force to rotational operation of a steering wheel, and is designed to adjust the assist force through power supply control which changes the amount of current flowing through the electric motor.
Such an electric power steering apparatus uses a battery as a power source. In consideration of anomaly of a power supply line, an apparatus disclosed in Japanese Patent Application Laid-Open (kokai) No. 2004-17732 includes two batteries; i.e., a main battery and an auxiliary battery, which are of the same voltage. The apparatus is configured such that when an anomaly has occurred in a power supply line connected to one battery, power supplied from the other battery is used through changeover.
However, the conventional apparatus requires a changeover device such as a changeover switch for selectively using power supplied from the two batteries, resulting in an increase in cost. Further, an electric power steering apparatus is required to generate a large drive torque, and therefore, use of a high-voltage-type battery has recently been considered. However, when a system using a high-voltage-type battery as a power source is configured, provision of another high-voltage-type battery as a backup for power failure becomes necessary. Therefore, cost of a power source for an electric power steering increases considerably, which is unfavorable.